The present invention is directed toward mechanical and/or chemical-mechanical planarization of microelectronic substrates. More specifically, the invention is related to planarizing machines with alignment systems for aligning optical monitoring systems with a microelectronic substrate during a planarizing cycle.
Mechanical and chemical-mechanical planarizing processes (collectively xe2x80x9cCMPxe2x80x9d) remove material from the surface of semiconductor wafers, field emission displays or other microelectronic substrates in the production of microelectronic devices and other products. FIG. 1 schematically illustrates a rotary CMP machine 10 with a platen 20, a carrier assembly 30, and a planarizing pad 40. The CMP machine 10 may also have an under-pad 25 attached to an upper surface 22 of the platen 20 and the lower surface of the planarizing pad 40. A drive assembly 26 rotates the platen 20 (indicated by arrow F), or it reciprocates the platen 20 back and forth (indicated by arrow G). Since the planarizing pad 40 is attached to the under-pad 25, the planarizing pad 40 moves with the platen 20 during planarization.
The carrier assembly 30 has a head.32 to, which a substrate 12 may be attached, or the substrate 12 may be attached to a resilient pad 34 positioned between the substrate 12 and the head 32. The head 32 may be a free-floating wafer carrier, or the head 32 may be coupled to an actuator assembly 36 that imparts axial and/or rotational motion to the substrate 12 (indicated by arrows H and I, respectively).
The planarizing pad 40 and the planarizing solution 44 define a planarizing medium that mechanically and/or chemically-mechanically removes material from the surface of the substrate 12. The planarizing pad 40 can be a fixed-abrasive planarizing pad in which abrasive particles are fixedly bonded to a suspension material. In fixed-abrasive applications, the planarizing solution is typically a non-abrasive xe2x80x9cclean solutionxe2x80x9d without abrasive particles. In other applications, the planarizing pad 40 can be a non-abrasive pad composed of a polymeric material, (e.g., polyurethane), resin, felt or other suitable non-abrasive materials. The planarizing solutions 44 used with the non-abrasive planarizing pads are typically abrasive slurries that have abrasive particles suspended in a liquid.
To planarize the substrate 12, with the CMP machine 10, the carrier assembly 30 presses the substrate 12 face-downward against the polishing medium. More specifically, the carrier assembly 30 generally presses the substrate 12 against the planarizing liquid 44 on the planarizing surface 42 of the planarizing pad 40, and the platen 20 and/or the carrier assembly 30 move to rub the substrate 12 against the planarizing surface 42. As the substrate 12 rubs against the planarizing surface 42, material is removed from the face of the substrate 12.
CMP processes should consistently and accurately produce a uniformly planar surface on the substrate to enable precise fabrication of circuits and photo-patters. During the construction of transistors, contacts, interconnects and other features, many substrates develop large xe2x80x9cstep heightsxe2x80x9d that create highly topographic surfaces. Such highly topographical surfaces can impair the accuracy of subsequent photolithographic procedures and other processes that are necessary for forming sub-micron features. For example, it is difficult to accurately focus photo patterns to within tolerances approaching 0.1 micron on topographic surfaces because sub-micron photolithographic equipment generally has a very limited depth of field. Thus, CMP processes are often used to transform a topographical surface into a highly uniform, planar surface at various stages of manufacturing microelectronic devices on a substrate.
In the highly competitive semiconductor industry, it is also desirable to maximize the throughput of CMP processing by producing a planar surface on a substrate as quickly as possible. The throughput of CMP processing is a function, at least in part, of the ability to accurately stop CMP processing at a desired endpoint. In a typical CMP process, the desired endpoint is reached when the surface of the substrate is planar and/or when enough material has been removed from the substrate to form discrete components on the substrate (e.g., shallow trench isolation areas, contacts and damascene lines). Accurately stopping CMP processing at a desired endpoint is important for maintaining a high because the substrate assembly may need to be re-polished if it is xe2x80x9cunder-planarized,xe2x80x9d or components on the substrate may be destroyed if it is xe2x80x9cover-polished.xe2x80x9d Thus, it is highly desirable to stop CMP processing at the desired endpoint.
In one conventional method for determining the endpoint of CMP processing, the planarizing period of a particular substrate is determined using an estimated polishing rate based upon the polishing rate of identical substrates that were planarized under the same conditions. The estimated planarizing period for a particular substrate, however, may not be accurate because the polishing rate or other variables may change from one substrate to another. Thus, this method may not produce accurate results.
In another method for determining the endpoint of CMP processing, the substrate is removed from the pad and then a measuring device measures a change in thickness of the substrate. Removing the substrate from the pad, however, interrupts the planarizing process and may damage the substrate. Thus, this method generally reduces the throughput of CMP processing.
U.S. Pat. No. 5,433,651 issued to Lustig et al. (xe2x80x9cLustigxe2x80x9d) discloses an in-situ chemical-mechanical polishing machine for monitoring the polishing process during a planarizing cycle. The polishing machine has a rotatable polishing table including a window embedded in the table. A polishing pad is attached to the table, and the pad has an aperture aligned with the window embedded in the table. The window is positioned at a location over which the workpiece can pass for in-situ viewing of a polishing surface of the workpiece from beneath the polishing table. The planarizing machine also includes a light source and a device for measuring a reflectance signal representative, of an in-situ reflectance of the polishing surface of the workpiece. Lustig discloses terminating a planarizing cycle at the interface between two layers based on the different reflectances of the materials. In many CMP applications, however, the desired endpoint is not at an interface between layers of materials. Thus, the system disclosed in Lustig may not provide accurate results in certain CMP applications.
Another optical endpointing system is a component of the Mirra(copyright) planarizing machine manufactured by Applied Materials Corporation of California. The Mirra(copyright) machine has a rotary platen with an optical emitter/sensor and a planarizing pad with a window over the optical emitter/sensor. The Mirra(copyright) machine has a light source that emits a single wavelength band of light.
U.S. Pat. No. 5,865,665 issued to Yueh (xe2x80x9cYuehxe2x80x9d) discloses yet another optical endpointing system that determines the endpoint in a CMP process by predicting the removal rate using a Kalman filtering algorithm based on input from a plurality of Line Variable Displacement Transducers (xe2x80x9cLVDTxe2x80x9d) attached to the carrier head. The process in Yueh uses measurements of the downforce to update and refine the prediction of the removal rate calculated by the Kalman filter. This downforce, however, varies across the substrate because the pressure exerted against the substrate is a combination of the force applied by the carrier head and the topography of both the pad surface and the substrate. Moreover, many CMP applications intentionally vary the downforce during the planarizing cycle across the entire substrate, or only in discrete areas of the substrate. The method disclosed in Yueh, therefore, may be difficult to apply in some CMP application because it uses the downforce as an output factor for operating the Kalman filter.
One concern of monitoring a planarizing cycle using an optical system that directs a light beam through a window in a polishing pad is that the window in the pad may not be aligned with the light source. For example, in web-format systems that slide a polishing pad over a table either during or between planarizing cycles, the pad may skew from side-to-side causing a window in the pad to become misaligned with a light source under the table. As such, it would be desirable to compensate for movement of the pad relative to the light source.
The present invention is directed toward planarizing machines, alignment systems for planarizing machines, and methods for planarizing microelectronic substrates using mechanical and/or chemical-mechanical planarization. In one aspect of the invention, a planarizing machine for mechanical and/or chemical-mechanical planarization of a microelectronic substrate comprises a table, a planarizing pad, and a substrate carrier. The table can have a support panel and an opening through the support panel. The planarizing pad is on the support panel, and the pad has a window aligned with the opening. The substrate carrier assembly has a carrier head configured to hold a microelectronic substrate and drive system coupled to the carrier head. The carrier head and/or the table are movable relative to each other to rub the substrate against the planarizing pad.
The planarizing machine also comprises an alignment assembly having a carriage assembly alignable with the opening and an actuator assembly coupled to the carnage assembly. The carriage assembly can have an emission site configured to be coupled to an optical monitoring system for directing a source light along a light path projecting from the carriage. Additionally, the actuator assembly is configured to move the carriage assembly relative to the window and the opening to align the light path with the window in the pad.
Another aspect of the invention is a method of planarizing a microelectronic substrate comprising: pressing a microelectronic substrate against a planarizing surface of a planarizing pad having an optically transmissive window; moving the microelectronic substrate and/or the planarizing pad relative to each other to rub the microelectronic substrate against the planarizing surface during at least a portion of a planarizing cycle such that the microelectronic substrate periodically passes over the window; monitoring a parameter of the planarizing cycle by directing a source light along a light path through the window in the planarizing pad and receiving a return light reflecting from the microelectronic substrate; and moving the light path from a first position to a second position relative to a movement of the window.